We are not yet married but been going out for 8 years. After about 2 years, all the lies about money and gambling came out. First and only time I ever saw him cry. He hated himself for it, and I know that now.
We talked about it and there was a lot of anger on my side. Is that the only thing he lied about? If so, then he has basically hidden his addiction and the debt because he is probably ashamed and never meant to hurt you but of course, he did. It's so hard.
We went to gamblers anonymous and I went with him and one thing I learnt is not not make him feel bad about the past, that is what they tell you at the support groups for family members.
You have to learn to accept what's happened and not look back and by doing so, you are his support system. My fiancé got badges for a week, a month, 2 months, 1 year gamble free etc and continues.
There is a register which will put him on some sort of list that will stop him from all sort of online gambling (that where my fiancé gambled, only online),
6 years later, he hasn't gambled once since it's happened, he's brilliant with money and I cannot see it ever happening again, with all addictions, it could always relapse, so I keep an eye on our finances but I couldn't even remember it was ever a thing when I read your post.
I hope you're ok. X